Lignin is a natural polymer, which can be extracted from e.g. wood. As lignin is a natural biopolymer its use as a component in e.g. glues instead of synthetic materials has been investigated in order to come up with more environmentally friendly applications. Further, it has been attempted to fractionate lignin according to the molecular weight of lignin in order to come up with lignin with specific properties to be used in specific applications. For example ultrafiltration, nanofiltration and sequential precipitation have been used for fractionating lignin.
Alkaline solutions such as alkaline sodium hydroxide solutions have been used in relation to the ultrafiltration process. However, the use of an alkaline solution increases the ash content in the lignin solution. This affects the further processing of the lignin and results e.g. in need of diafiltration or of using additional sulphuric acid in order to obtain lignin fractions suitable for further processing. Thus the procedure for fractionating lignin becomes complicated and costly.
The inventors have therefore recognized a need for a more simple and effective method, which would result in fractionated lignin and especially in fractionated lignin having a reduced content of inorganic material and thus having suitable properties for use in different applications.